1. Field of the Invention
The present invention relates to a buried object detection sensor, and in particular to a buried object detection sensor performing a detection of a buried object from the ground by a combined sensor.
2. Description of the Related Art
A detection device using a metal detector(MD) or a ground penetrating radar (GPR) is used to detect a buried object from the ground. The detection device using the MD has a coil which transmits/receives a magnetism, and is suitable for a detection of an object buried in a relatively shallow depth. Meanwhile, the detection device using the GPR has an antenna which transmits/receives an electromagnetic wave, and is suitable for a detection of an object buried in a deep portion compared to the case when the MD is used. A combined type detection device using both the MD and the GPR is therefore proposed in which both characteristics of the MD and the GPR can be obtained.
As this combined type detection device, an invention according to Japanese Patent publication No. 2001-242263 can be cited. The combined type detection device, disclosed in this Japanese Patent publication No. 2001-242263, has a constitution in which a ring-shaped detection coil to be the MD, and an antenna to be the GPR at a center side of the detection coil, are included. The detection coil generates a magnetic field when a current is supplied, to make a metal object buried underground generate an induced electromotive force. The detection coil senses the magnetic field generated by the induced electromotive force, to thereby probe a presence/absence of a buried object. Besides, the GPR generates an electromagnetic wave from the antenna when the current is supplied. The buried object reflects this electromagnetic wave, and therefore, the antenna senses a reflected electromagnetic wave to probe the presence/absence of the buried object. As stated above, the detection of the buried object is performed by using both sensors in a combined state.
In the combined type detection sensor using the MD and the GPR, when it has a constitution in which a transmission coil of the MD is provided at a center of the GPR and a receiving coil is provided at a periphery of the GPR, an eddy current is generated in the GPR caused by the magnetic field generated at the transmission coil when the current is supplied to the transmission coil. The eddy current forms a loop at all over the GPR, and therefore, a magnetic flux is generated from the GPR, then the magnetic flux works in a direction to deteriorate a performance of the MD. Namely, the magnetic flux generated at the GPR is received by the receiving coil of the MD, and therefore, the receiving coil becomes to have an offset value. The receiving coil has a constant dynamic range, and therefore, an SN ratio may fall if a rate of change of the magnetic field by the buried object is detected under a state having the offset value. A receiving level of the MD in a case of the combined type detection device using the MD and the GPR becomes to be a tenth part or less when the receiving level of the detection device using only the MD is assumed to be one.